Single cell point to multipoint (SC-PTM) transmission is transmission of a multicast service on a physical downlink shared channel (PDSCH). Evolved multimedia broadcast/multicast service (eMBMS) transmission and unicast transmission cannot reuse a resource in a same subframe, and subframe configuration in a multicast-broadcast single-frequency network (MBSFN) region to which eMBMS transmission is applied cannot be dynamically adjusted. Consequently, a resource allocated for eMBMS transmission is wasted. An SC-PTM transmission technology can reduce resource waste occurred in eMBMS transmission, and improve spectral efficiency.
Different from a PDSCH that carries unicast data, a PDSCH that carries multicast data in an SC-PTM transmission process is referred to as a multicast PDSCH. In the SC-PTM transmission process, a base station sends a physical downlink control channel (PDCCH) to a group of user equipments (UE) in a cell. The group of UEs obtain scheduling information of the multicast PDSCH by receiving the PDCCH, then receives the multicast PDSCH according to the scheduling information, and further obtains the multicast data carried on the multicast PDSCH.
Similar to unicast transmission, SC-PTM transmission also supports a hybrid automatic repeat request (HARQ) technology. A base station sends, in a downlink subframe to a group of UEs, a multicast PDSCH that carries multicast data. Some or all UEs in the UE group send, in an uplink subframe to the base station, hybrid automatic repeat request-acknowledgement (HARQ-ACK) information used to feed back the multicast PDSCH. When the UEs successfully receive the multicast PDSCH, the UEs feed back an acknowledgement ACK to the base station. When the UEs fail to receive the multicast PDSCH, the UEs feed back a negative acknowledgement (NACK) to the base station. After receiving the HARQ-ACK feedback information, the base station learns of a multicast data receiving status of the UEs according to the HARQ-ACK feedback information, and then determines whether the multicast PDSCH that carries the multicast data needs to be retransmitted to the UEs.
UE that supports SC-PTM transmission can simultaneously receive, in a same subframe, one unicast transmission and N multicast PDSCH transmissions sent by a base station, and N is a positive integer. Therefore, the UE needs to feed back, to the base station, multiple pieces of HARQ-ACK feedback information specific to the multiple transmissions. However, it is not defined in an existing HARQ feedback mechanism that UE feeds back multiple pieces of HARQ-ACK feedback information (including multicast HARQ-ACK feedback) to a base station.